In a known spark-ignition type internal combustion engine, an adjustment of ignition timing is used together with an adjustment of an amount of air as means for controlling torque thereof. In the art disclosed in JP-A-2005-113877, for example, a demanded torque is corrected by ignition timing efficiency determined according to a difference between base ignition timing and an MBT and, based on the efficiency-corrected demanded torque, a demanded throttle opening degree is calculated. In addition, an estimated torque is obtained in the MBT estimated from an actual amount of air and an engine speed and an ignition retard amount is calculated relative to the MBT based on a ratio between the estimated torque and the demanded torque before the correction.
According to the art disclosed in the above publication, the throttle opening degree changes according to a change in the demanded torque and the amount of air changes according to the change in the throttle opening degree. The estimated torque then changes according to the change in the amount of air. Specifically, the estimated torque changes, following the demanded torque. Various types of response lags such as, for example, a lag in arithmetic operations performed by a control system, and signal transmission carried out within a control system, a lag in a throttle operation, or a lag in a sensor output are produced during a process in which the demanded torque is reflected in the estimated torque. There is, as a result, a time deviation at all times between the estimated torque and the demanded torque.
The abovementioned time deviation poses a problem when the demanded torque is changing transiently, or particularly when the demanded torque is decreasing. For example, when the demanded torque is changing vibratingly, the estimated torque follows the demanded torque to change vibratingly. The abovementioned various types of response lags appear, at this time, as a phase shift between the estimated torque and the demanded torque. This results in periodically occurring a period of time during which the estimated torque is greater than the demanded torque.
According to the art disclosed in the above publication, the ignition retard amount is determined according to the ratio between the estimated torque and the demanded torque, so that the ignition timing is retarded more than the MBT during the period in which the estimated torque is greater than the demanded torque. Such ignition retarding is automatically performed even when the ignition timing efficiency is set to the maximum efficiency, that is, when the operation in the MBT is demanded. Specifically, the art disclosed in the above publication involves possible unnecessary aggravation of fuel efficiency as a result of unintentional retarding of the ignition timing.
Relating to the above problem, the applicant proposed an invention that served as a solution thereto in Japanese Patent Application No. 2008-178963. According to the invention of this application, when a change rate in the demanded torque can be achieved through a throttle operation, the torque efficiency that serves as the basis for setting the ignition timing is fixed to 1 as the maximum efficiency. Should the estimated torque become greater than the demanded torque as a result of the time deviation between the estimated torque and the demanded torque, therefore, unnecessary ignition retarding can be prevented because of the torque efficiency fixed to the maximum efficiency.
If, however, the demanded torque itself changes at a rate that cannot be achieved by the throttle operation only, the ignition retarding is activated. Assume, for example, a case in which a periodically increasing or decreasing component is added to the demanded torque for the purpose of, for example, inhibiting vehicle pitching. Fuel efficiency is aggravated, though the change rate in the demanded torque can be achieved thanks to the activation of the ignition retarding. Fuel efficiency is particularly important among other performance requirements of a vehicle. To give improved fuel efficiency the top priority, retarding of the ignition timing should be inhibited as much as feasible.